Vehicles are known which include a die-cast sub-frame mounted rearward of a power plant (engine/transmission unit), and left and right suspension arms supported at left and right ends of the sub-frame. The sub-frame includes a hollow body having a hollow part thereinside formed by using a core, a front wall inclined upwardly toward the rear of a vehicle body, and a rear wall inclined upwardly toward a front of the vehicle body. Such a vehicle sub-frame is disclosed, for example, in Japanese Patent Application Laid-Open Publication (JP-A) No. 2012-136195.
In the sub-frame disclosed in JP 2012-136195A, since the front wall is inclined upwardly toward the rear of the vehicle body and the rear wall is inclined upwardly toward the front of the vehicle body, the sub-frame has a substantially trapezoidal shape in vertical cross section with a position where a fixed die and a movable die for casting meet together, i.e. a parting line position, being located at a bottom part of the sub-frame.
It is preferred that the sub-frame has a constant thickness so as to suppress an increase in weight. To form the sub-frame with the constant thickness, it is required that a core used to form a hollow part inside the sub-frame has a substantially trapezoidal shape in vertical cross section. The core is formed by solidifying casting sand with a binder. When the core is formed in the substantially trapezoidal shape in vertical cross section, the core has acute angle parts. The acute angle parts are fragile, and thus it is difficult to secure strength of the core.
To solve this problem, it is possible that the core is formed in a substantially hexagonal shape in vertical cross section with the acute angle parts being removed to thereby secure strength of the core. However, the sub-frame has the substantially trapezoidal shape in vertical cross section. If the core having the substantially hexagonal shape in vertical cross section is used, the sub-frame has parts thicker than other parts, and it becomes difficult to suppress the increase in weight of the sub-frame.
It is therefore an object of the present invention to provide a die-cast vehicle sub-frame which is capable of suppressing an increase in weight of the sub-frame itself while securing strength of a casting core.